The Science and Practice of Photography

PHOTOGRAPHY match of the negatives at C and E the more reliable are the settings. To read total density adjust D so that the illumination matches; then insert the plate at C and move A till the illumination matches again, and the scale reading will give the total density.

To read the density less fog, glass, and gelatine, place the unexposed spot in C while making the first setting of D and make the second setting as above and the scale reading will be the density less fog, glass, and gelatine.

If the density of the spot which it is desired to read is so great that A can not be moved near enough to C to make a match, it becomes necessary to substitute a spot of intermediate density. Read its density as above, and with all parts in the final positions of the reading, move A back to zero.

Readjust for equality by moving D and if necessary changing the negative E, and proceed with the reading of the original spot. In this way as many stops may be put between as are necessary to bring the final reading on the scale, and the density of the spot in question will be the sum of this series of differences. With each shift of the lamp, A, back to zero, the brightness of the image falls off, thus setting a limit to the series unless brighter sources can be substituted.

Let i be the brightness of the image when matched